1. Field of the Invention
The present invention relates to communication network signalling systems, and more particularly to apparatus for monitoring and testing links within a communication network signalling system.
2. History of the Prior Art
Various different signalling systems are used in communication networks such as telephone networks. Originally, most signalling systems were of the in-band type in which detection of an off hook condition resulted in a switch securing a trunk, and then a line, and so forth. Signalling was accomplished on voice trunks. In time, in-band signalling systems proved to be inefficient, among other reasons because a call to a telephone that turned out to be busy tied up a considerable amount of equipment unnecessarily. This led to the advent of common channel or out-of-band signalling systems.
Most present day telephone networks employ common channel signalling systems which provide a high speed digital network. Such systems employ a number of common components including service switching points (SSPs) which comprise local switches for initiating signalling and for generating the signalling system protocol. Signal transfer points (STPs) serve to route the messages. Service control points (SCPs) comprise databases which are used in such systems. In some signalling systems, each service switching point is connected to two signal transfer points. Such dual or redundant connections prevent the loss of a single node due to fault or error conditions from rendering much or all of the network inoperative.
In spite of the dual or redundant interconnections of components within most common channel signalling systems, such systems are not without monitor and test problems. The protocol of such signalling systems includes message signal units (MSUs) which, among other functions, provide switching instructions in the event of a fault. In certain situations, a minor fault or failure has been known to produce a large number of maintenance signalling units which proceed to flood the network and shut down much of the entire system. Such catastrophic failures have shut down telephone networks covering large portions of major metropolitan areas, in recent years.
Within common channel signalling systems, the various components are connected by various different connections or links. For example, a signal transfer point may be coupled by links to a plurality of D4 channel banks, which in turn are coupled to other components. It is desirable to be able to monitor conditions within the various links. To do this, the signal transfer point is sometimes coupled to the channel banks through an integrated digital service terminal (IDST) which is a multiplexor/demultiplexor capable of providing electronic cross-connections. This type of connection provides test access for monitoring and testing the links, but with the disadvantage that the integrated digital service terminal is connected in series with and becomes an active element within the link. As such, problems within the integrated digital service terminal can directly affect the link.
It is also common practice to couple an integrated digital service terminal as an active element within the links between signal transfer points. In such arrangements, the links are used to couple one signal transfer point to the integrated digital service terminal through DSO dataports at an input side of the integrated digital service terminal. The integrated digital service terminal which has a test access interface and a matrix for providing desired interconnections is also provided with T1 cards at an output side thereof for coupling via links to a second signal transfer point.
In addition to integrated digital service terminals, still other components are sometimes coupled in the link between a signal transfer point and a D4 or similar channel bank. For example, intelligent patch panels are sometimes included. Such patch panels may include a keyboard controlled terminal for automatic connection to a selected one of a plurality of jacks so that test equipment can be properly interconnected.
The frequent connection by links of a signal transfer point to D4 or similar channel banks in signalling systems has emphasized the need to monitor and test such links in view of the fact that channel banks such as the D4 channel banks tend to be relatively trouble prone. However, it would be desirable to be able to advantageously monitor and test links at any location within a signalling system. Ideally, a link monitor and test arrangement should be a passive element which is basically transparent to the link and which does not interfere with link operation, except when the need arises to enter the series connection of the link for purposes of in-depth testing. Such apparatus should provide a test interface capable of monitoring the link in a non-obtrusive manner while at the same time being capable of inserting signals in series with the link for in-depth link testing and analysis.